Method and apparatus for fabricating circuit-forming-substrate and circuit-forming-substrate material

ABSTRACT

A method for manufacturing a printed wiring board, comprising the step of forming a hole by an energy beam such as a laser beam, wherein formation of a resin film by a substrate-material resin oozing to the inner-wall surface of a hole is prevented, by lowering the water-absorption percentage of a substrate material through the dehumidifying step as the preprocess of the hole-forming step for forming a through-hole or non-through-hole for interconnecting circuits formed on both sides or in multiple layers, thereby it is possible to realize high-quality hole-formation by preventing a defective resin film formation and obtain a high-reliability printed wiring board.

This is a divisional of application Ser. No. 09/355,455 filed Sep. 15,1999, now U.S. Pat. No. 6,409,869.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a qualityprinted wiring board of high reliability using an energy beam, such as alaser beam, for hole-formation. Manufacturing equipment and boardmaterial for the printed wiring board are also included in theinvention.

BACKGROUND ART

In order to meet the needs in the recent electronic appliance industryfor mounting components in high density in a compact space, theincreasing number of printed wiring boards is shifting from theconventional single-sided version towards the double-sided and thosehaving multi-layers. Thus the development efforts are focused onproducing a high density printed wiring board that can accept as manywiring circuits thereon.

For forming a fine hole (approximately 200 μm diameter, for example) ata high speed, use of an energy beam, such as a laser beam, is beingstudied in place of the conventional through-hole making with a drill(Kiyoshi Takagi: “Significant Trends, the Development of BuildupMultilayered PWBs”, Surface Mounting Technology, No.1, pages 2-10(1997); and other publications). Examples of the board material includea film of thermosetting resin at B-stage state containing uncuredcomponent, and a composite material of woven, or nonwoven, organic orinorganic fabric and thermosetting resin.

A conventional process comprises a hole forming step and a connectionmeans forming step, as shown in FIG. 5. The aim of forming a hole in awiring board is to interconnect the circuits provided on the surfaces orin the inner layers of a wiring board. The interconnection means isformed by filling the hole with a conductive paste or by applying aplating process. For example, a both-surface type printed wiring boardis produced by first filling a through-hole of the board with aconductive paste containing electroconductive particles by way of aprinting or the like process, and then copper foil is provided on bothsurfaces of the board to be unitized by thermal compression. The copperfoils are then made to have specific patterns. The board material isrequired to become soft or molten at the thermal compression; so, aB-stage thermosetting resin containing uncured component or athermoplastic resin is used.

When forming a hole in a wiring board of the above described materialwith an energy beam, such as a laser beam, a resin 2 and a woven, ornonwoven, fabric 3 contained in the board material 1 are heated by thebeam and sublimated to be scattering around, and a hole 5 is formed.Resin material existing around the hole is also softened and molten by aheat of low-energy beam that does not contribute to perforating a hole,which resin material oozing out from the wall surface of the hole formsa thin film to cover a part or the whole of the wall surface, asillustrated in FIG. 6(a).

In a case when the resin material of the board absorbs moisture, it issoften and molten more easily by the heat of an energy beam, and volumeof the oozing resin increases. As volume of the absorbed moistureincreases, the oozing resin comes into contact to each other as shown inFIG. 6(b), which is then solidified and contracted after cooling to forma thin resin film 10, eventually clogging the through-hole 5. Thisphenomenon becomes significant with the holes of small diameter.

FIG. 7 is a chart showing a concept of relationship between percentageof water absorbed in the board material 1 and rate of rejects due toformation of resin film 10. As soon as the percentage of water absorbedin the board material (water absorbed versus board material 1 in termsof weight; hereinafter referred to as wt %) exceeds a certain point thereject rate sharply picks up. The reject rate and the percentage ofabsorbed moisture in the threshold value vary depending on the holediameter and the board material 1. In the through-holes 5 having such aresin film 10 formed therein, troubles in the electrical interconnectionoften arise in a subsequent process step. Examples of the troubleinclude; no electrical connection accomplished between the circuits ofboth surfaces or layers of a wiring board because of incompleteformation of the conductive material or the plating through the oppositesurface of a board, the hole not filled with sufficient amount ofconductive material, a high contact resistance in an accomplishedinterconnection, etc.

The present invention aims to offer a manufacturing method, amanufacturing equipment and a board material for producing reliableprinted wiring boards having quality holes. In accordance with theinvention, rejects due to the formation of resin film is eliminated, orat least reduced, to an improved production yield rate.

DISCLOSURE OF THE INVENTION

The present invention relates to a method for manufacturing a printedwiring board. The method comprises steps of forming a through-hole, or anon-through-hole, with an energy beam in a board material of plate form,or sheet form, composed mainly of either a thermoplastic resin or athermosetting resin containing uncured component, or a mixture of both,or in a board material of plate form, or sheet form, made of a woven, ornonwoven, fabric impregnated with a material composed mainly of eitherthe thermoplastic resin or thermosetting resin containing uncuredcomponent, or a mixture of both, forming connection means in thethrough-hole, or nonthrough-hole, provided by the hole-forming step, forelectrically interconnecting one surface and the reverse surface of theboard material, and dehumidifying the board material before thehole-forming step.

Under the manufacturing process in accordance with the presentinvention, production yield rate is improved through a reduced rate ofrejects due to the formation of resin film during the hole-forming step.A high quality hole formation is realized and a high-reliability printedwiring board is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing the process in accordance with a firstexemplary embodiment of the present invention.

FIG. 2(a)-(f) are cross sectional views illustrating the process ofmanufacturing a printed wiring board in accordance with the firstembodiment of the present invention.

FIG. 3(a)-(c) show process flowcharts in accordance with a secondexemplary embodiment of the present invention.

FIG. 4(a), (b) show laminating processes in accordance with a thirdexemplary embodiment of the present invention.

FIG. 5 is a flowchart showing a conventional process.

FIG. 6(a), (b) show cross sectional views of a hole formed by aconventional procedure.

FIG. 7 is a characteristics chart showing the relationship between therate of rejects due to formation of resin film and the percentage ofwater absorbed in board material.

BEST MODE FOR CARRYING OUT THE INVENTION

A method for manufacturing a printed wiring board in accordance with thepresent invention comprises the steps of forming a through-hole, or anon-through-hole, with an energy beam in a board material of plate form,or sheet form, composed mainly of either a thermoplastic resin or athermosetting resin containing uncured component, or a mixture of both,or in a board material of plate form, or sheet form, made of a woven, ornonwoven, fabric impregnated with a material composed mainly of thethermoplastic resin or thermosetting resin containing uncured component,or a mixture of both, forming connection means in the through-hole, ornon-through-hole, provided by the hole-forming step for electricallyinterconnecting one surface and the reverse surface of the boardmaterial, and dehumidifying the board material before the hole-formingstep. With the above method of manufacture, rate of rejection due to theformation of resin film at the hole-forming step is reduced, andhigh-quality hole-formation is realized to obtain a high-reliabilityprinted wiring board.

Preferably, the dehumidifying step is a hot air drying process. Thereby,a board material of high moisture absorption rate can be surelydehumidified with ease.

Preferably, temperature of the hot air drying process is 50° C. orhigher, and not higher than a limitation of temperature and time periodin which the gel time of thermosetting resin does not start shifting.Thereby, a board material of high moisture absorption rate can be surelydehumidified with ease in a relatively short period of time, withoutinviting a change in the material property of the board material.

Preferably, the dehumidifying step is a vacuum drying process. Thereby,a board material of high moisture absorption rate can be surelydehumidified with ease.

Preferably, the vacuum drying is performed without accompanying aheating. Thereby, a board material of high moisture absorption rate canbe surely dehumidified easily, without inviting a change in the materialproperty of the board material.

Preferably, the vacuum drying process is accompanied by a heating.Thereby, a board material of high moisture absorption rate can bedehumidified efficiently within a short period of time.

Preferably, the heating temperature is at least lower than boiling pointof a solvent contained in a resin of board material, and lower than alimitation of temperature and time period in which the gel time ofthermosetting resin does not start shifting. Thereby, a board materialof high of moisture absorption rate can be dehumidified without invitingbubbling of impregnated resin which might be caused by evaporation ofsolvent, and without inviting a change in the material property of theboard material.

Preferably, degree of the vacuum ambient is not lower than 100 Torr.Thereby, the dehumidifying is conducted efficiently within a shortperiod of time.

Preferably, the dehumidifying is performed at once with two or morepieces of board materials stacked together direct to each other.Thereby, the plurality of board materials may be efficientlydehumidified, which is a substantial advantage in volume production.

Preferably, the dehumidifying step brings the content of water in theboard material to be not larger than 1% in terms of weight percentage.Thereby, rejects due to the formation of resin film is prevented, andhigh-quality hole-formation is realized. The reliability level of aprinted wiring board is improved.

A method for manufacturing a printed wiring board in accordance with thepresent invention comprises the steps of forming a through-hole, or anon-through-hole, with an energy beam in a board material of plate form,or sheet form, composed mainly of either a thermoplastic resin or athermosetting resin containing uncured component, or a mixture of both,or in a board material of plate form, or sheet form, made of a woven, ornonwoven fabric, impregnated with a material composed mainly of eithersaid thermoplastic resin or thermosetting resin containing uncuredcomponent, or a mixture of both, forming connection means in saidthrough-hole, or non-through-hole, provided by said hole-forming stepfor electrically interconnecting one surface and the reverse surface ofthe board material, and suppressing moisture absorption of said boardmaterial before said hole-forming step. With the above method ofmanufacture, moisture absorption during storage on shelf can besuppressed; as a result, rejects due to the formation of resin film inthe hole-forming step is reduced, and high-quality hole-formation isrealized. The reliability of a printed wiring board is improved.

Preferably, the step of suppressing moisture absorption is provided ineither between the dehumidifying step and the hole-forming step or inthe hole-forming step, or in both steps. By so doing, moistureabsorption in the hole-forming step is also suppressed.

Preferably, the process of suppressing the moisture absorption isconducted by storing a board material on shelf in a low humidityatmosphere. By so doing, the absolute humidity goes down and thepercentage of water content in a board material is lowered.

Preferably, ambient temperature of the hole-forming step is higher thandew point temperature of the low humidity atmosphere. Under suchenvironment, there will be no dew condensation on a board material.

Preferably, the low humidity atmosphere is a vacuum ambient. Under suchstate, the absolute humidity can easily be lowered and the percentage ofwater content of a board material can be lowered.

Preferably, the vacuum ambient is a hermetically sealed space which isevacuated after a board material is lodged in. By so doing, the absolutehumidity can easily be lowered and maintained.

Preferably, the low humidity atmosphere is a hermetically sealed, or acirculating, space substituted with dry nitrogen. This provides a simplemethod for suppressing the moisture absorption during storage on shelf.

Preferably, the process of suppressing moisture absorption keeps contentof water in a board material to be 1% or lower in terms of weightpercentage. By so doing, rejects due to the formation of resin film isprevented, and high-quality hole-formation is realized. The reliabilitylevel of a printed wiring board is improved.

Preferably, the low humidity atmosphere is an atmosphere in which thepartial pressure of water vapor is not larger than 10 mmHg. Under suchatmosphere, content of water in the board material after storage onshelf never goes higher than 1% in terms of weight percentage,regardless of the time period it stayed on shelf.

A method for manufacturing a printed wiring board in accordance with thepresent invention comprises the steps of forming a through-hole, or anon-through-hole, with an energy beam in a board material of plate form,or sheet form, composed mainly of a thermoplastic resin or athermosetting resin containing uncured component, or a mixture of both,or in a board material of plate form, or sheet form, made of a woven, ornonwoven, fabric impregnated with a material composed mainly of saidthermoplastic resin or thermosetting resin containing uncured component,or a mixture of both, and forming connection means in said through-hole,or non-through-hole, provided by said hole-forming step for electricallyinterconnecting one surface and the reverse surface of a board material,wherein said board material stored on shelf is carried to thehole-forming step within a time period in which the water content insaid board material does not go beyond 1% in terms of weight percentage.Under the above described manufacturing method, the moisture absorptionmay be suppressed only through the simple control of time.

Preferably, the board material is laminated at least on both surfaceswith sheet film having a high anti-moisture permeability until it iscarried to the hole-forming step. By so doing, the moisture absorptioncan be suppressed regardless of the absolute humidity of storageenvironment.

Preferably, the board material is laminated at both surfaces with filmsheets having a high anti-moisture permeability whose size is greaterthan the shape of board material, and said film sheets covering the bothsurfaces are sealed together by heat in the area exceeding outer edge ofthe board material so that the board material is isolated from theoutside air until it is carried to the hole-forming step. By so doing,moisture absorption of a board material from the end can also besuppressed during storage on shelf.

Preferably, the sheet film having a high anti-moisture permeability isof polyethylene terephthalate. It is inexpensive and recyclable.

Preferably, the film is provided with a metal film formed on the surfaceby deposition or the like method. The use of such a film suppresses themoisture absorption during storage on shelf more effectively.

Preferably, the metal film is of aluminum. It is inexpensive.

Preferably, the energy beam is a laser beam. The laser beam has a goodconcentration property on a board material, and it can be scanned easilywith an optical device or the like means.

Preferably, the laser beam is a CO₂ laser beam. It provides a highenergy at low cost.

Preferably, the process of filling the through-hole, ornon-through-hole, provided by the hole-forming step with a pastecontaining conductive particles is included in the connection meansforming step. By so doing, rejects due to the formation of resin film inthe hole-forming step can be reduced, and high-quality hole-formation isrealized to an improved reliability of a printed wiring board.

Preferably, the process of providing a board material filled with apaste containing conductive particles with a metal foil on one surface,or both surfaces, to be unitized together by thermal compression isincluded in the connection means forming step. By so doing, rejects dueto the formation of resin film in the hole-forming step can be reduced,and high-quality hole-formation is realized to an improved reliabilityof a printed wiring board.

Preferably, the process of providing the through-hole, ornon-through-hole, formed by the hole-forming step with a plating ofconductive metal is included in the connection means forming step. By sodoing, rejects due to the formation of resin film in the hole-formingstep can be reduced, and high-quality hole-formation is realized to animproved reliability of a printed wiring board.

Preferably, the thermosetting resin is an epoxy resin. It has animproved anti-moisture property.

Preferably, the woven, or nonwoven, fabric is made mainly of organicfiber material. The use of organic fiber, which has a relatively closematerial property to the resin, makes the hole-formation with an energybeam easy.

Preferably, the organic fiber material is mainly of an aromaticpolyimide fiber. It makes the hole-formation with an energy beam easy,and the printed wiring board lighter in weight and more reliable.

An equipment for manufacturing a printed wiring board in accordance withthe present invention forms a through-hole, or non-through-hole, in aboard material of printed wiring board with an energy beam generatedfrom an energy beam generating unit, wherein the dehumidifying means isprovided in a section for feeding said board material. With the abovedescribed manufacturing equipment, rejects due to the formation of resinfilm in the hole-forming step is reduced, and high-qualityhole-formation is realized to an improved reliability of a printedwiring board.

Preferably, the dehumidifying means is a hot air dryer. Thereby, a boardmaterial of high moisture absorption can be surely dehumidified withease.

Preferably, temperature of the hot air drying is not lower than 50° C.,and not higher than a limitation of temperature and time period in whichthe gel time of board material does not start shifting. Thereby, a boardmaterial of high moisture absorption rate can be surely dehumidifiedwith ease in a relatively short period of time, without inviting achange in the material property of the board material.

Preferably, the dehumidifying means is a vacuum drying unit. Thereby, aboard material of high moisture absorption rate can be surelydehumidified with ease.

Preferably, the vacuum drying unit does not accompany a heating.Thereby, a board material of high moisture absorption rate can be surelydehumidified with ease, without inviting a change in the materialproperty of the board material.

Preferably, the vacuum drying unit accompanies a heating. Thereby, aboard material of high moisture absorption rate can be dehumidifiedefficiently within a short period of time.

Preferably, heating temperature of the vacuum drying unit is set to beat least lower than boiling point of a solvent contained in resin of theboard material. Thereby, a board material of high moisture absorptionrate can be dehumidified without inviting a bubbling of the impregnatedresin, which bubbling is caused by an evaporating solvent, and a changein the material property of the board material.

Preferably, vacuum degree in the vacuum drying unit is set to be notlower than 100 Torr. Thereby, a board material is surely dehumidifiedefficiently within a short period of time.

Preferably, the dehumidifying means is a unit that is capable ofdehumidifying two or more pieces of board materials at once stackingtogether direct to each other. Thereby, the plurality of board materialsmay be efficiently dehumidified, which is a substantial advantage in thevolume production.

Preferably, the dehumidifying means is a unit that is capable ofreducing the content of water absorbed in the board material to be lowerthan 1% in terms of weight percentage. Thereby, rejects due to theformation of resin film is prevented, and high-quality hole-formation isrealized to an improved reliability of a printed wiring board.

An equipment for manufacturing printed wiring board in accordance withthe present invention forms a through-hole, or non-through-hole, in aboard material for forming printed wiring board with an energy beamgenerated from an energy beam generating unit, wherein means forsuppressing moisture absorption is provided either in a section forfeeding said board material or in a section for forming the hole, or inboth sections. With the above described manufacturing equipment, rejectsdue to the formation of resin film in the hole-forming step is lowered,and high-quality hole-formation is realized to an improved reliabilityof a printed wiring board.

Preferably, in a manufacturing equipment for forming a through-hole, ornon-through-hole, in a board material for forming printed wiring boardwith an energy beam guided from an energy beam generating unit, meansfor suppressing the moisture absorption is provided either in a sectionfor feeding said board material or in a section for forming the hole, orin both sections. By so doing, the moisture absorption during thehole-forming step can also be suppressed.

Preferably, the means for suppressing moisture absorption is a unit thatstores a board material on shelf in a low humidity environment.

Preferably, the low humidity environment is a vacuum state. In thevacuum state, the absolute humidity goes low and the percentage of watercontent of a board material can be reduced.

Preferably, the vacuum state is a sealed space which is evacuated aftera board material is lodged in. In this state, the absolute humidity caneasily be lowered and maintained there.

Preferably, the low humidity environment is a hermetically sealed space,or a circulating environment, substituted with dry nitrogen. Thisprovides a simple method of suppressing the absorption of moistureduring storage on shelf.

Preferably, the means for suppressing moisture absorption is a unit thatkeeps the content of water in a board material to be lower than 1% interms of weight percentage. With such means, rejection due to theformation of resin film is prevented, and high-quality hole-formation isrealized to an improved reliability of a printed wiring board.

Preferably, the low humidity environment is an environment where thepartial pressure of water vapor is not larger than 10 mmHg. Under suchenvironment, the content of water in a board material never goes higherthan 1% in terms of weight percentage, regardless of the time period itstays on shelf.

A material for manufacturing printed wiring board in accordance with thepresent invention is a board material of plate form, or sheet form,composed mainly of a thermoplastic resin or a thermosetting resincontaining uncured component, or a mixture of both, or a board materialof plate form, or sheet form, made of a woven, or nonwoven, fabricimpregnated with a material composed mainly of said thermoplastic resinor thermosetting resin containing uncured component, or a mixture ofboth, wherein content of water in said board material being lower 1% interms of weight percentage. With the above described material, rejectsdue to the formation of resin film in the hole-forming step areavoidable, and high-quality hole-formation is realized to an improvedreliability of a printed wiring board.

Preferably, the board material for manufacturing printed wiring board isthe one that fits to the hole-forming with an energy beam. With such amaterial, rejects due to the formation of resin film in the hole-formingstep are avoidable, and high-quality hole-formation is realized to animproved reliability of a printed wiring board.

Preferably, the thermosetting resin is an epoxy group resin. It has animproved anti-moisture property.

Preferably, the woven, or nonwoven, fabric is made mainly of organicfiber material. The use of organic fiber, whose material property isrelatively close to the resin, makes the hole-forming with an energybeam easy.

Preferably, the organic fiber material is mainly of an aromaticpolyimide fiber. It makes the hole-forming with an energy beam easy, anda printed wiring board lighter in weight and more reliable.

Now in the following, exemplary embodiments in accordance with thepresent invention are described referring to FIG. 1 through FIG. 4.

(Embodiment 1)

FIG. 1 and FIG. 2, respectively, are process flowchart and crosssectional views showing a method for manufacturing printed wiring boardin accordance with a first exemplary embodiment of the presentinvention. The manufacturing process is composed of a dehumidifyingstep, a hole-forming step and a connection means formation step, in theorder. A board material 1 is a composite material of, as shown in FIG.2(a), a thermosetting resin 2 (an epoxy resin, for example) and anonwoven fabric 3 of aromatic polyamide fiber (hereinafter referred toas aramid fiber). The thermosetting resin 2 is not completely cured, butit is in the so-called B-stage state containing non-cured component. Theboard material 1 is a one which is normally called “prepreg”. The boardmaterial 1 of approximately 150 μm thick is dehumidified, and athrough-hole 5 is formed in a laser processing unit with a CO₂ gas laserbeam 4 at a diameter of approximately 200 μm.

The through-hole 5 is filled with a conductive paste 7 composed mainlyof an epoxy resin and conductive particles by a printing method, asshown in FIG. 2(c). Then, as shown in FIG. 2(d), a metal foil 8 isprovided on both surfaces of the board material 1 to be unitizedtogether by thermal compression in the direction of thickness. The metalfoils 8 overlaid on the both surfaces are electrically interconnected bythe conductive paste 7. Each of the metal foils 8 is etched to a certainspecific pattern to complete a printed wiring board, FIG. 2(f).

Some of the points of significance with the present embodiment aredescribed below. The dehumidifying is conducted either by an independentdehumidifying unit or dehumidifying means provided in a section of thelaser processing unit for feeding board material 1. In practice, a boardmaterial 1 that has a high moisture absorbing property may bedehumidified with hot air or vacuum prior to the hole-formation.

In the dehumidifying with hot air, it has been confirmed that higher thehot air temperature the higher the dehumidifying effect, and thedehumidifying process can be finished within a short period of time whenthe temperature is 50° C. or more. However, in order to prevent adverseinfluence to the following thermal compression process, it is essentialthat the board material 1 is treated within a limitation of temperatureand time period in which the gel time of thermosetting resin 2 containedin the board material 1 does not make a substantial shift. The term,“gel time”, is a parameter representing the progress of curing; it isdefined by the length of time: A thermosetting resin 2 of a certainspecific weight is placed on a hot plate heated to a certain specifictemperature (170° C., for example), stirred with a Teflon or the likestick, a period of time until the thermosetting resin 2 ceases to rope.Normally the dehumidifying with hot air is conducted with theatmospheric air, but the use of dry nitrogen, for example, may lead to amore effective result.

The dehumidifying in vacuum has an advantage that it can dehumidify aboard material in the room temperature, without applying heat. Thus, aboard material 1 may be dehumidified without inviting any shift in thematerial property, such as the gel time. If a heat is added during thevacuum dehumidifying, the effect is enhanced and the time needed fordehumidifying will become shorter as compared with the time required inthe drying with hot air. Attention has to be paid in order not to leaveadverse influence on the thermal compression process; that it has to beprocessed within a limitation of temperature and time in which the geltime of thermosetting resin 2 contained in the board material 1 does notmake a substantial shift.

When the thermosetting resin 2 contains a solvent, the upper limit ofthe heating temperature should be lower than boiling point of thesolvent. If the temperature is not lower than the boiling point, solventevaporates to result in a bubbling of the thermosetting resin 2, whichleaves ill affect to a subsequent process step. Another advantage withthe dehumidifying with vacuum is that it can dehumidify at once two ormore pieces of board materials 1 of a sheet form stacked together directto each other. Thus, the vacuum dehumidifying takes only a short periodof time for dehumidifying, and provides a high operational efficiency.The process is suitable to the volume production. The process ofdehumidifying the board materials 1 in a stacked state is not suitableto the hot air drying, because the dehumidification effect is small inthat way and takes too much time, allowing the gel time of boardmaterial 1 to shift significantly. It has been confirmed that the vacuumdehumidifying brings about a satisfactory dehumidification effect withina short period of time if grade of the vacuum is not lower than 100Torr. If the vacuum grade is inferior to the value, it takes too muchtime for dehumidification, or hardly any dehumidification is obtainable.

The lower the moisture contents in a board material 1, which representsthe result of the dehumidifying process, the lower the rate of rejectsdue to the formation of resin film 10. According to experimental resultsobtained by the inventor, the rejects are eliminated when the moisturecontent is not higher than 1 wt % (measured with a Karl Fisher moisturemeter). However, as the quantity of thermosetting resin 2 contained inthe board material 1 increases, the rejects rate due to formation resinfilm 10 tends to go higher. So, the moisture content in a board materialshould ideally be almost zero. In practice, the moisture content shouldpreferably be 0.6 wt % or less, taking the dispersion in the materialproperty of board material 1 into consideration. More preferably, itshould be 0.2 wt % or less when taking into account the stability in avolume production line.

(Embodiment 2)

FIG. 3 shows process flowcharts of a printed wiring board in accordancewith a second exemplary embodiment of the present invention. The processconsists of means for suppressing moisture absorption, a hole-formingstep and a connection means forming step. There are variations to theprocess; a case of FIG. 3(a), where the means for suppressing moistureabsorption is placed before the hole-forming step, a case of FIG. 3(b),where the means is placed in between the dehumidifying step ofembodiment 1 and the hole-forming step, a case of FIG. 3(c), where themeans is in the hole-forming step, and a case where the cases of FIG.3(b) and FIG. 3(c) are combined. Process steps after the hole-formingstep are basically the same as those of embodiment 1, therefore detaileddescription of which steps is omitted.

The points of significance with the present embodiment are describedbelow. The means for suppressing moisture absorption is constituted byan independent moisture suppressing unit, or means for suppressingmoisture absorption disposed in a section for feeding the board material1 or in the hole-forming step of a laser process unit. Practically, thepresent embodiment is suitable for such board materials 1 whose initialabsorption of moisture is relatively low. The moisture absorption can besuppressed by storing the board materials on shelf in a low humidityatmosphere. An air conditioner is generally used to provide the lowhumidity atmosphere, A simpler alternative may be providing ahermetically sealed space, or a circulating environment, substitutedwith dry nitrogen. Or, the board material 1 may be placed in acontainer, and then the container is evacuated after it is hermeticallysealed.

In a case where there is a certain time period for a board material 1dehumidified by the process of embodiment 1 until it is carried to thehole-forming step, it may be stored on shelf in the low humidityatmosphere for suppressing the moisture absorption. It is furthereffective if the hole-forming step is conducted in a low humidityatmosphere. The point of importance here is that, if environment of thehole-forming step is not low humidity, temperature of the low humidityatmosphere needs to be kept higher than dew point of the environment ofthe hole-forming step, in order to avoid dew condensation on the boardmaterial 1.

Although moisture absorption with a board material 1 is low in thebeginning, or it had been dehumidified to a sufficiently low level, itstarts absorbing moisture if stored in a certain environment, eventuallyreaching a certain percentage of water content, which is proportionateto absolute humidity of the atmosphere in which the board material isstored. Experiments conducted by the inventor reveal that the percentageof water content can be maintained below 1 wt % so long as it is storedon shelf in an environment where the partial pressure of water vapor isnot higher than 10 mmHg. The moisture absorption never go beyond 1 wt %so long as it is stored in the above described environment, regardlessof time span of the storage.

Another practical way of forming a hole in a board material 1 in a statewhere the moisture contents are not higher than 1 wt % is to processingit before it absorbs moisture to a level higher than 1 wt % duringstorage on shelf. In this way, means for suppressing the moistureabsorption is not required; it is a simple manufacturing method whichrequires only the control of time.

(Embodiment 3)

Other method for suppressing the moisture absorption of board material 1is to laminate the both surfaces with a sheet film having a highanti-moisture permeability. Laminating the board material 1 at least onboth surfaces with sheet film 11 of anti-moisture permeability, asillustrated in FIG. 4(a), suppresses the moisture absorption. Theanti-moisture effect is enhanced by using sheet films 11 that have agreater size than the board material 1, and sealing them together byheat in an area outer than the board material 1. By so doing, the boardmaterial 1 is isolated from the outside air and the moisture absorptionfrom the end is also suppressed. Polyethylene terephthalate (PET), whichis inexpensive and recyclable, may be a good candidate for the film 11of anti-moisture permeability. The effectiveness of moisture suppressionis further enhanced if the sheet film is provided with a film ofaluminum, or the like metal, deposited on the surface.

Although in the above described three exemplary embodiments theallowable contents of moisture in the board material have been specifiedto be not higher than 1 wt %, effectiveness of the suppressed moistureabsorption remains as it is in reducing the rejects due to formation ofresin film 10 even if the moisture absorbed is somewhat more than theabove specified value. Although CO₂ laser has been described as anexample of the energy beam, other gas lasers, YAG laser and othersolid-state lasers, eximer lasers, or energy beams other than laser mayof course be used instead for the purpose. Although the descriptionshave been made referring to a double-faced printed wiring board, thepresent technology is of course applicable to the manufacture ofmulti-layered printed wiring boards by repeating the process steps.Instead of the nonwoven fabric, a woven fabric may also be used; thenonwoven fabric or woven fabric may also be formed with an organic fiberother than aramid fiber or with an inorganic fiber material such asglass; a thermoplastic resin may also be used in place of thethermosetting resin; the connection means may be provided also by aninterconnection formed with plating or pressure welding. Thus, theabove-described embodiments are not to be interpreted as limiting.

INDUSTRIAL APPLICABILITY

Providing a dehumidifying step before the hole-forming step brings abouta substantial advantage in improving the manufacturing yield ratethrough the reduced rejects due to formation of resin film. Ahigh-quality hole-formation is realized in a printed wiring boardmanufactured in accordance with the present invention, and the printedwiring boards thus manufactured are furnished with a high-reliability.

In addition to the above-described advantages, following effects areexpected.

As the diameter of a hole goes smaller the oozing resin readily stickstogether, which leads to the rejects due to formation of resin film. Thepresent invention advances the smallest limit of the hole diameter astep further. The high-quality hole-formation is realized with the holesof smaller diameters. This technology helps present high-density printedwiring boards at a high manufacturing yield rate.

Furthermore, as the quantity of resin oozing out from the wall surfaceof a hole decreases the quantity of conductive paste to be filled in thehole increases accordingly, and the compression rate of conductiveparticles increases in the thermal compression procedure. Thus, highreliability printed wiring boards having low interconnection resistanceare presented.

1. A printed wiring board material selected from the group consisting ofa first material of plate form or sheet form comprising a thermosettingresin, and a second material of plate form or sheet form comprising awoven or nonwoven fabric comprising an organic fiber materialimpregnated with a thermosetting resin, wherein said printed wiringboard material has a water content of not more than 0.6% by weight andis one in which holes can be formed with an energy beam.
 2. The materialof claim 1, wherein the thermosetting resin is an epoxy resin.
 3. Thematerial of claim 1, wherein the organic fiber material comprises anaromatic polyamide fiber.